Air filtering material



Patented July 18, 1944 AIR FILTERING MATERIAL Irving Newton Smith,Granville, Ohio, assignor to Owens-Corning Fiberglas Corporation, acorporation of Delaware No Drawing. Application February .11, 1943,Serial No. 475,573

3 Claims.

This application is a continuation-in-part of my copending applicationSerial No. 419,620, filed November 18, 1941.

The dust catching and retaining material or so-called adhesivepreviously used in air filters was usually petroleum oil. Although thismaterial is efiective as a dust catcher it is inflammable, therebycreating a fire hazard. In the case of a filter having a base offibrousor shredded material, the fire hazard is increased due to thefinely divided nature of the oil on the base.

It has been attempted to overcome these objections by using very heavyoils which are less inflammable although these, too, are combustiblewhen exposed to flame. The heavy oils, however, because of their highviscosity, wet dust particles slowly and as a result the dust collectingefiiciency of the filter was seriously lowered. It has also beensuggested to add a fire-proofing material to low viscosity oil to securenon-inflammability. Many of the truly flame-proof compounds which couldbe added to such oils are solids so that the resulting flame-proof oilwas of too great a viscosity. i

An object of the present invention is to provide a filter adhesive freefrom the objections to previous materials. The material of the presentinvention is non-inflammable, odorless and inexpensive and has a largecapacity for holding dust.

It is a further object of the present invention to provide a completelynon-combustible dust catching filter material as distinguished from themerely non-inflammable materials heretofore employed. I obtain thisobject by providing a dust catching material in an aqueous system, thematerial consisting mainly of an inorganic substance. The combination ofthis material with an inorganic filter base such as glass wool providesa truly fireproof filter. It also provides a readily washable filtersince the aqueous system and adhering dust may be easily rinsed from thebase with water and without deleterious effect on the wool. The base maybe then retreated by the application of fresh adhesive.

It is another object of the invention to provide an aqueous system as adust catching adhesive that will maintain proper viscosities at allrelative humidities to which the filter may be subjected in use. Theadhesive is prevented from reaching such a low viscosity at highrelative humidities that it will flow from the porous base, and fromreaching at low relative humidities such high viscosities that it nolonger is fully effective to catch dust particles.

I have discovered that a dust catching material having the desirableproperties of non-combustibility, good dust catching efiiciency, freedomfrom odor and low cost, may be obtained by employing a deliquescentinorganic substance that forms an aqueous solution of relatively lowvapor pressure, thereby providing a permanent liquid that willeffectively catch and retain dust particles.

Very extensive tests have shown that zinc chloride is particularlysuited to the purpose of the present invention because it forms a watersolution having a very low vapor pressure. The vapor pressure of suchsolution is approximately as low as the vapor pressure of atmosphericmoisture at 5% relative humidity within the range of temperaturesnormally encountered by the filters in use. A humidity as low as 5% isvery seldom, if ever, met with in the conditions under which air filtersare employed so that the filter adhesive retains its liquidity undersubstantially all humidity conditions to which it may be subjected.

An aqueous solution of zinc chloride has considerable surface tension sothat it wets most dust particles with difiiculty and, therefore, is notfully effective as a filter adhesive for some types of dust. This isovercome in the present invention by adding a wetting agent to the zincchloride solution. This wetting agent may be either a neutral wettingagent or a cation-active wetting agent. The anionic wetting agents havenot been found suitable since they are precipitated by the zincchloride.

An acid-stable neutral wetting agent that has been found verysatisfactory is a modified mannitol monolaurate which is obtainableunder the trade name of Atlas G-9046T. This may be replaced if desiredby sorbitol monolaurate or by cation-active wetting agents of the typeused in the dyeing of textiles or by other suitable weting agents.

An adhesive of an aqueous solution of zinc chloride becomes more diluteas the relative humidity of the air passing through the filterincreases. At high relative humidities of about to the adhesive may takeup so much water from the air passing through the filter that theviscosity drops 50 low that the adhesive flows tions. As the zincchloride solution becomes more dilute such a material causes aprogressive increase of viscosity so that the tendency of the adhesiveto fiow from the fibrous base at higher relative humidities is oflset.

Several materials including carbohydrates, proteins, resins, gum andsoaps have been found suitable to some extent for this purpose but testshave shown that cellulose is by far the most efficient material forcontrolling the viscosity of the zinc chloride adhesive and is effectiveover the widest range of relative humidity of the air being filtered.

In aqueous solutions of zinc chloride of about 60% or greaterconcentration, cellulose fibers swell and finally dissolve. Since alarge proportion of the cellulose molecules are of colloidal size, amarked increase in viscosity results, arfd under some conditions even agel may be formed. If a small quantity, say 0.5%, of cellulose bedissolved in 70 B. zinc chloride under mild conditions, that is lowacidity, low temperature, and short time heating, a very viscoussolution is obtained; a solution too viscous to apply to filter pads bymost methods of application. If this solution be gradually diluted withwater, it is observed that the first additions oi. water cause a markeddecrease in viscosity. When the solution reaches a concentration of zincchloride in the order of 55%, it begins to gel and at 50% is a verystifi gel. A very small addition of water beyond this point causescomplete coagulation of the cellulose and liberation of the non-viscouszinc chloride solution. This means that with this adhesive on an airfilter, a relative humidity as low as about 50% would be sufiiclent tocause bleeding of the zinc chloride solution from the filter.

The explanation of this gelation and coagulation of the cellulose upondilution of the zinc chloride lies in the fact that the cellulose issoluble in concentrated zinc chloride, but not in water. In order toretain gel properties without coagulation at higher dilutions, (andhence higher humidities of the air passing through the filter) thesolubility of cellulose in water must be increased. Increasing thesolubility of cellulose, however, means shortening its chain length, andtherefore reducing its viscosity and gel strength, which reductions areopposed to the results required, namely, an increase in the viscosity ofthe filter adhesive.

It is possible to obtain the results desired, how-:

ton on the basis of the zinc chloride solution at 70 B. The amount ofcellulose may be varied from 1 to 4% depending upon the humidityconditions to be encountered in use of the filter. To obtain solution ofthis much cellulose in the zinc chloride it is desirable to use thefollowing procedure.

The 70 B. zinc chloride solution is first acidified by the addition ofhydrochloric acid suflicient to make the solution about .1% hydrochloricacid. To this may be added the wetting agent in proportions of about 3parts wetting agent to 1000 parts zinc chloride solution in the case ofmodified mannitol monolaurate. The acidified solution is then heated toabout 80 C., and 3% by weight of cotton is added with vigorousagitation. The temperature is held at 80 C. for a time sufiicient toobtain complete solution of all the cotton in the zinc chloride, whichis usually five minutes. This time may vary with the temperature towhich the solution is heated and with the amount of cotton added, andalso with the acidity of the solution. Reduction in temperature and/oracidity will generally increase the time of heating. A longer heatingperiod may be advantageous in some cases because the time ls not then ascritical and less accurate control need be maintained.

At the end of the heating period the solution is neutralized andpreferably made slightly basic by adding sumcient sodium hydroxide tobring the zinc oxide content to about .3%. The mixture is then quicklycooled. It may be further diluted with water, if desired, for ease inapplication to the fibrous or other porous base. If the material is tobe sprayed onto the fibrous base, it is preferable to dilute it to about50% to 70%. preferably 60%, zinc chloride.

The adhesive of the present invention employed with a base of mattedglass fibers forms a completely fireproof dust-catching filter suitablefor filtering air in domestic and commercial heating and airconditioning systems and in other installationsswherever dust of anynature is to be removed from a gaseous medium.

Various modifications of my invention may be made within the scope ofthe claims.

I claim:

1. Filter material comprising a porous base oi fibrous glass of aporosity permitting the ready passage of air therethrough, and a coatingthereon of zinc chloride in water solution, cellulose, and a wettingagent.

2. A filter material comprising a porous base of siliceous material, anda coating thereon of a 50% to aqueous solution of zinc chloride, awetting agent, and about 3% cellulose on the basis of the zinc chlorideat 70 B.

3. A filter for gas comprising a base of glass wool, and a dust-catchingadhesive on the base containing a 60% aqueous solution of zinc chloride,about .3% mannitol monolaurate, and about 28% 1gglluloseon the basis ofthe zinc chloride at IRVING NEWTON SMITH.

